The uniqueness in the operation of a cognitive radio is that a cognitive radio is able to sense the spectral environment over a wide frequency band and exploit this information to provide wireless links that best meet a user's communications requirements. In this context, the cognitive radio does not have primary access rights to the frequency band used. As such, it is necessary that the cognitive radio is able to dynamically detect the presence of the signals transmitted by the primary users, so that it can avoid transmitting signals in the frequency channels used by the primary users.
Energy detection is a commonly used method for sensing or detecting the presence of signals transmitted by the primary users. Unlike coherent detection, energy detection does not need any information on the signal to be detected. As such, energy detection is robust against unknown multi-path fading.
However, energy detection is vulnerable to noise uncertainty, because this method requires accurate knowledge of noise power. In this regard, there are many causes of noise uncertainty, for example, non-linearity of components, thermal noise in components (which is non-uniform and time varying), and noise due to signal transmissions from other users. Accordingly, it is virtually impossible in practice to obtain accurate knowledge of noise power.